Circadian rhythms are nearly ubiquitous endogenous timing systems that help coordinate the myriad physiological, metabolic and developmental processes that occur continuously in each organism at all times of the day. This circadian clock is comprised at the molecular level of interlocked and autoregulatory feedback loops that are built from complex interactions that are constantly changing in relation to each throughout the 24 hour cycle. The long term goal of this project is to understand the functional relationships among the post- transcriptional processes that keep the circadian oscillator running. We are using genetic, genomic, biochemical and cell biological tools and strategies of the model plant Arabidopsis to identify the molecules and mechanisms that regulate the transport of important proteins and mRNA between the cytosol and nucleus. We are focused in part on how a range of post-translational modifications of clock proteins affect both their positional and temporal intracellular localization. Gatekeeping features of the intracellular environment will also be addressed, both on a genomic scale and at the level of a single molecular species. We are applying for the first time in circadian studies single cell imaging techniques using a photo switchable fluorescent protein to assess features of clock protein movement and turnover that will be applicable to non-plant circadian systems. We are also exploiting certain plant-specific advantages of small RNA processing to address other post- transcriptional control mechanisms of the circadian clock. Taken together our program will probe mechanisms of circadian control that should be broadly applicable across all eukaryotic systems.